Because the air in the country is really clean, we ought to live there much as possible. Since, however, a great deal of the world’s (1) work must be done indoor in cities, it is important that we take every (2) precaution to ventilate our houses properly. Some people have thought that night air is injurious. But careful study shows that night air is identical with which we breath during the day. In fact the (3) proper ventilation of a bedroom is one of the first necessity for good (4) health. Since the exhaled air is usually warmer and lighter than the inhaled air, it rises to the top of the room. Therefore it is better to open a window both at the top to let the warm up air out and also at (5) the bottom to admit the fresh air in. Of course, this does not mean (6) that one should sleep in a strong draft. In many places it is feasible to sleep out-of-the-doors on a sleeping porch and so to secure perfect (7) ventilation. In recent years we have seen steady progress made in the development of equipments to supply proper conditioned air not only in large (8) auditoriums, class-rooms, and factories, but also in railroad trains and in private homes. This equipment cleans the air off dust, keeps (9) the temperature comfortable, holds the humidity at the right point, and keeps the air in the motion. Such a condition is conductive to (10) efficiency as well as good health.